Variability in quantitative outcomes of instrumental swallowing assessments in adults: a scoping review

ABSTRACT Purpose To map scientific evidence on the variability of quantitative parameters extracted by instrumental swallowing assessment tests in adults, using the coefficient of variation. Research strategies The methodological procedures recommended by the Joanna Briggs Institute and the extension for scoping reviews of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA-ScR) were followed. Selection criteria The search was carried out in the Pubmed/Medline, Lilacs, Cochrane Library, Embase, Web of Science, Scopus and CINAHL databases, as well as in Google Scholar to consult the gray literature. Data analysis Two blind and independent reviewers screened the articles by title and abstract. Subsequently, the articles were read in full and selected according to the eligibility criteria. Data were extracted according to a standardized instrument. Results 363 studies were found, 13 of which were eligible. Most studies had a sample size of less than 30 participants and were made up of healthy individuals. The instrumental exams used were diverse: videofluoroscopy, electrical impedance tomography, laryngeal sensors, high-resolution manometry and surface electromyography. The studies searched for intra-individual variability and the coefficient of variation ranged from low to high variability, as the instruments, parameters and collection procedures were very heterogeneous and non-standardized. Conclusion Intra-individual variability of the quantitative outcomes of instrumental swallowing assessments in adults ranged from low to high according to the exam, outcome, presence or absence of underlying disease, consistency and volume of the bolus.


INTRODUCTION
Swallowing is a complex function that involves multiple structures and neuromuscular regions (1,2) .The initial clinical evaluation consists of structural and functional observation to collect data, observe signs and symptoms, and guide clinical reasoning (3) .However, in isolation, it may be inefficient to detect non-visible changes that characterize a swallowing disorder (4) , compromising the effectiveness of treatment (5) .
Therefore, instrumental examinations are important to investigate these parameters more precisely (6) .Structural details are analyzed by observing the images from these examinations, obtaining more quantitative data -which have been gaining prominence in light of technological advances (7) because they aid in the diagnostic process (5,8) .
In clinical practice, the videofluoroscopy swallowing study (VFSS) provides real-time visualization of food transport through sequential video radiographic images (9) .It is considered the reference procedure for evaluating and identifying the risk or occurrence of food penetration or aspiration (9)(10)(11)(12) , identifying swallowing disorders, and analyzing the effects of treatment strategies (13)(14)(15) .
Videoendoscopy is also used to detect changes in swallowing.This exam provides visualization of the hypopharynx and larynx, thus enabling the observation of residues, penetration, and aspiration in the laryngopharyngeal region (15) .It can be performed in both children and adults (16) and is feasible for quantitative analysis of the duration of the pharyngeal phase (10) .
Ultrasonography, in turn, has been used as a complement in the investigation and monitoring of morphometric and kinematic parameters of the oral and pharyngeal phases of swallowing (17,18) .Its applicability in the face of technical innovations helps diagnose and treat dysphagia, enabling, for instance, evaluations of hyolaryngeal and tongue movement (19,20) .There are also exams such as high-resolution manometry that can reveal changes in pressure and time measurements in the pharynx and esophagus during swallowing, even if there are no complaints or apparent clinical signs (6) .
Due to the complexity of extracting and analyzing quantitative measures, most exams require trained and experienced evaluators (21)(22)(23) because it is difficult to standardize methods.Moreover, there are intrarater and interrater particularities that make it impossible to compare studies.However, these exams make it possible to investigate aspects related to swallowing performance through images and quantify data (8) .Thus, quantification enables the comparison and monitoring of the patient's evolution, providing complementary and guiding information for the therapeutic process (24) .
The reliability analysis and validity of assessment instruments in the literature are extremely important to verify the quality of diagnostic information (25) .Also, the variability analysis of quantitative data extracted by these tests provides an important contribution.Variability or dispersion is normally investigated using indicators such as the coefficient of variation (CV), calculated by the ratio between the standard deviation and the mean of the data set (26) .
CV is a dimensionless measure of variability; therefore, it compares data with different units, and the result is given in percentages (27) .Normally, the CV of biological systems ranges from 10-15% (28) ; when the CV is above 30%, it indicates that the measure has high heterogeneity (29) .Thus, the lower the CV, the lower the degree of variability (30) .
Dispersion can be influenced by various biological and assessment factors.For instance, instrumental swallowing assessment patterns may differ in the time interval between offers, food consistency, volume, and sequence of attempts (31) .By analyzing the variability of the measures, one can understand their homogeneity and representativeness, which helps to identify inconsistencies (32) .Thus, it can be analyzed whether these measures actually help to characterize the parameter of interest and thus decide whether they are safe to be applied in practice.
Hence, this review aimed to map the scientific evidence on the variability of quantitative parameters extracted by instrumental swallowing exams through the investigation of the CV.The study was guided by the following research question: "What is the level of variability of quantitative parameters of instrumental swallowing assessments in adults?".

METHOD
This scoping review was conducted in accordance with the methodological recommendations of the Joanna Briggs Institute (JBI) for the type of study in question (33) and followed the criteria of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis -Extension for Scoping Reviews (PRISMA-ScR) (34) .The protocol for this review was previously published and reported the objectives, eligibility criteria, sources, search strategy, selection, analysis, and data presentation methods (35) .It was registered in the Open Science Framework on January 28, 2023 (https://osf.io/p3g2e/).The PCC acronym was used to define the research question, as follows: Population (young and/or older adults who underwent swallowing assessment), Concept (level of variability of quantitative data resulting from the swallowing assessment), and Context (studies using instrumental exams with quantitative results of the CV for the analysis of swallowing parameters).

Search Strategies
The bibliography was surveyed on December 3, 2022, and an updated search was performed on November 30, 2023, in the databases of PubMed/MEDLINE, LILACS via BVS, Cochrane Library, EMBASE, Web of Science, Scopus, and CINAHL via EBSCO.The gray literature was also searched in Google Scholar, considering only the first 100 articles retrieved in the search.It was not possible to perform the search in the ProQuest database as provided for in the protocol (35) since the authors' institution was not granted access at the time of the search.
The search strategy was based on the combination of descriptors and keywords (Chart 1) adapted for each database.The references in retrieved articles were also considered and manually checked to identify studies that could be relevant to the topic of interest.

Study Selection
After the search, the study had the following stages: 1.The articles recruited in each database were imported into EndNote software (Clarivate Analytics, PA, USA) for management and removal of duplicates; 2. Two reviewers used Rayyan software (Qatar Computing Research Institute, Doha, Qatar) to blindly screen articles by title and abstract; 3. Conflicts were resolved through analysis by a third reviewer.
The study included all articles available in full in the established sources of evidence, without restrictions on year or language.It excluded studies that did not analyze the level of variability through CV, that did not report the mean and standard deviation to enable the calculation of CV by the researchers, that used alternative formulas to calculate CV, that used instrumental examinations but did not present quantitative results, that evaluated only esophageal swallowing, and that approached only children.

Data extraction and presentation
The studies were selected for inclusion based on the steps presented in the flowchart recommended by PRISMA-ScR (34) .After the initial filters, the two reviewers analyzed the included articles by reading their full text, considering the eligibility criteria to maintain them in the final result.The data were extracted according to the research objectives, following an extraction matrix previously presented in the protocol of this review (35) .
The data extracted from the articles included in this review are shown in Chart 2. The studies were published between 1990 and 2022, and six of them (6,24,26,36,37,43) were developed in the United States.Six (46.15%) studies (30,31,36,37,39,40) had a sample of fewer than 30 participants.More than 50% of the studies had a mixed population in terms of sex (men and women) and age (young and older adults).Two articles (15.3%) mentioned pairing the sample by sex (6,43) .
The articles focused on using the CV to verify the intraindividual variability of quantitative measures in repetitions of the same swallowing task and compare intraindividual CVs between

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In general, the CVs followed a pattern: higher for G3, lower for G1, and intermediate for G2.

2017, Sweden
The original article presented the CV as the result of the ratio between mean and standard deviation.The calculation in % was performed by the authors of this review Park et al. (31) , The only significant difference in CV between MAS and LAS was in the electromyographic peak measurement, in the laryngeal region, during the swallowing of thin liquid Ohmori et al. (30) , Nevertheless, the studies analyzed different populations and parameters, although the consistency of the food bolus offered in the swallowing tasks was decisive for the oscillations in CV values.In any case, the data indicate that the measures obtained through VFSS, considered the reference standard for instrumental evaluation of swallowing, do not have wellestablished intraindividual or interindividual variability.
The studies included in this review have several differences that make comparison difficult, such as the lack of standardization in the nomenclature of consistencies, the diversified volumes, the different utensils, the number of repetitions, and the different parameters investigated.Another important aspect is the small sample sizes in these studies, commonly fewer than 30 participants, which especially compromises the external validity of the results.
These studies provide no robust information on sample calculation, sampling, or whether the CV was calculated in this process.The CV is considered a relatively reliable indicator of a repeated measure or task, being dependent on the proportional changes in the mean and standard deviation of the sample (heteroscedasticity) (44) .Therefore, the lack of more solid information on sample selection in these studies weakens the interpretation of the data and is a limitation to be overcome in future studies that analyze variability through CV calculation.different groups, whether by age, clinical condition, volumes, or food consistencies.CV values ranged from low to high variability, with quite heterogeneous and non-standardized instruments, parameters, and collection procedures.

DISCUSSION
This scoping review aimed to map the available evidence on the variability of quantitative measures obtained in instrumental swallowing examinations in adults.The results indicated that the studies prioritized the investigation of intraindividual variability and that the heterogeneity of the examinations, collection procedures, and quantitative parameters contributed to the high CV amplitude, which also limited the comparison between studies.
The review found studies with different types of instrumental swallowing examinations.Some were authored by researchers in common (6,39,40,43) , which may justify, in these cases, the occurrence of similar methods and instruments.High-resolution manometry and surface electromyography were the instruments most used in the selected studies.Although the literature indicates VFSS as the reference instrument for the evaluation of this function (10)(11)(12) , only two studies that investigated CV used this examination directly (36,42) .Most studies were conducted with healthy individuals.Those with an underlying disease were restricted to PD (6,24,43) , head and neck cancer (42) , or multiple diagnoses (38) .Hence, researchers have invested more in understanding the variability of quantitative measures obtained in instrumental swallowing tests in people with preserved swallowing function than in dysphagic individuals.
When comparing these groups, intraindividual variability was generally greater in patients with swallowing disorders (38) or a specific underlying disease (6) .It is known that intraindividual or interindividual variability may be influenced by individual factors such as sex, age, and anatomical and functional differences.It must be considered that the dispersion of data may be greater when there is also an adverse clinical condition, due to the body's natural adaptations.
Variability can be an important parameter for monitoring and early detection of dysphagic signs and symptoms.A study using high-resolution manometry that investigated pressure in different anatomical regions during swallowing found that pressure variability in the velopharynx helped to distinguish healthy individuals from those with early to intermediatestage PD (6) .It also found that an approach with swallowing assessment associated with complementary tests and protocols can indicate early changes in PD that are not observed in isolated evaluations.
Another example of a study in individuals with PD used surface electromyography (24) .Considering the population with PD and low levels of swallowing impairment, a significant difference was found in the variability of the amplitude of the electromyographic peak in the laryngeal region between the most affected side and the side less affected by the disease when swallowing thin liquids.The authors of the study believe that the smaller variation on the most affected side may be the result of the less force produced in the muscles involved in swallowing.
Most outcomes indicated the influence of the swallowed volume.For instance, the CV was higher in the semisolid swallowing task in most measures investigated in one of the studies that used VFSS (36) .The other study with VFSS analyzed the penetration aspiration scale (PAS) scores (42) .The data showed high variability for 20 ml of thin liquid, 5 ml of mildly thick liquid, and 3 ml of extremely thick liquid.Its authors attribute this high variability to the complexity required from the swallowing mechanism due to larger volumes and the need for multiple swallows in thicker consistencies.
The variability of parameters must be determined to understand which of them present better homogeneity in the repetitions analyzed, and which one is therefore the most reliable for evaluating and interpreting the performance of the investigated function.An example of this is a study that used surface electromyography, whose parameters with the lowest intraindividual variability among the various measures analyzed were offset latency, duration, and maximum amplitude of electromyographic activity during swallowing (31) .Another study revealed that variability helped to identify that a voluntarily modifiable swallowing method with instructions may be the most suitable to apply in surface electromyography in clinical practice (30) .
Nonetheless, the high intraindividual dispersion poses a challenge to determining values that represent the expected normal range of the quantitative parameters investigated.An example of this complexity is a study that used high-resolution manometry and found that pharyngeal motility generated pressures with different degrees of variability depending on the anatomical region (26) .
The different CV results found in each article reflect the heterogeneity of the methods adopted in the studies.The articles were concerned with using CV to verify intraindividual variability by repeating different swallowing tasks at least twice and comparing intraindividual CVs between different groups, whether by age, clinical condition, volume, or food consistencies.
Some limitations were found during this literature mapping.Few articles were eligible because some researchers investigated variability through measures other than CV or used it to evaluate morphometry, strength, resistance, coughing, or respiratory flow.Although these characteristics are associated with swallowing, they do not necessarily evaluate the individual performing this function.The heterogeneity of the methods restricted the comparison between studies and indicated the need for greater standardization of collection and analysis procedures, including the cutoff point for interpreting CV.

CONCLUSION
Studies that analyzed the variability of quantitative swallowing parameters obtained through instrumental examinations are heterogeneous and indicated that the dispersion of measures ranges from low to high according to the type of examination, parameter, presence or absence of underlying diseases, and characteristics of the food bolus such as consistency and volume.

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